MUSCLES
AND MOVEMENT

HOW MUSCLES WORK

A
voluntary muscles usually works across a joint. It is attached to both the
bones by strong cords called tendons.

When the
muscles contracts, usually just one bone moves.

For example
when the biceps in the arm contracts, the radius moves but the scapula does
not.

ORIGIN AND
INSERTION

When a muscle
contracts, usually just one bone moves. The other is stationary. The origin is
where the muscle joins the stationary bone. The insertion is where it joins the
moving bone. When a muscle contracts, the insertion moves towards the origin.

TENDONS

Tendons
are the cords and straps that connect muscles to bones. At the bone, the fibres
of the tendon are embedded in the periosteum of the bone. This anchors the
tendon strongly and spreads the force of the contraction, so the tendon wonít
tear away easily.

MUSCLE WORKING IN PAIRS

Muscles usually
work in pairs or groups,† e.g. the biceps
flexes the elbow and the triceps extends it.

This is
called antagonistic muscle action. The working muscle is called the prime mover
or agonist. (itís in agony!) The relaxing muscle is the antagonist. The other
main pair of muscle that work together are the quadriceps and hamstrings.†

The prime
mover is helped by other muscles called synergists. These contract at the same
time as the prime mover. They hold the body in position so that the prime mover
can work smoothly.

When muscles cause a limb to move through the joint's range of
motion, they usually act in the following cooperating groups:

agonists

These muscles cause the movement to occur.
They create the normal range of movement in a joint by contracting. Agonists
are also referred to as prime
movers since they are the muscles that are primarily responsible
for generating the movement.

antagonists

These muscles act in opposition to the
movement generated by the agonists and are responsible for returning a limb to
its initial position.

synergists

These muscles perform, or assist in
performing, the same set of joint motion as the agonists. Synergists are
sometimes referred to as neutralizers
because they help cancel out, or neutralize, extra motion from the agonists to
make sure that the force generated works within the desired plane of motion.

fixators

These muscles provide
the necessary support to assist in holding the rest of the body in place while
the movement occurs. Fixators are also sometimes called stabilizers.

TYPES
OF CONTRACTION

The
contraction of a muscle does not necessarily imply that the muscle shortens; it
only means that tension has been generated. Muscles can contract in the
following ways:

isometric contraction

This is a contraction in
which no movement takes place, because the load on the muscle exceeds the
tension generated by the contracting muscle. This occurs when a muscle attempts
to push or pull an immovable object.

isotonic contraction

This is a contraction in
which movement does
take place, because the tension generated by the contracting muscle exceeds the
load on the muscle. This occurs when you use your muscles to successfully push
or pull an object.

Isotonic
contractions are further divided into two types:

concentric contraction

This is a contraction in
which the muscle decreases in length (shortens) against an opposing load, such
as lifting a weight up.

eccentric contraction

This is a contraction in
which the muscle increases in length (lengthens) as it resists a load, such as
lowering a weight down in a slow, controlled fashion.

During a concentric contraction, the muscles
that are shortening serve as the agonists and hence do all of the work. During an
eccentric contraction the muscles that are lengthening serve as the agonists
(and do all of the work).